A three-dimensional structure determination of Southern Bean Mosaic Virus (SBMV) is being attempted using X-ray crystallography. SBMV is a small spherical virus of 6.4 X 10 to the 6th power MW with 22% RNA. There are 180 protein subunits, each of 29,000 MW, generating an icosahedral protein envelope with T equals 3 symmetry. Crystals with an R32 space group have been found suitable for high resolution studies. The virus particle sits on a special 32 position, signifying that the crystallographic asymmetric unit contains only one-sixth of the virus, the remainder of the particle can then be generated from the crystal symmetry. Data to 11A resolution has been collected. This is currently being extended to 6A resolution using a rotation camera and computer controlled film scanner. A survey for heavy atom derivatives is in progress. It is hoped to improve single isomorphous replacement phasing with molecular replacement by averaging between the independent protein subunits. It may be possible to observe some of the secondary structure both of the protein and the nucleic acid. However, the intention is to eventually determine the structure at 3A resolution, and thus to be able to state explicitly the position of each atom in this small virus.